Abstract
The life and thermal properties of plasma sprayed TBC – widely used in gas turbine engines – are closely related to the microstructure of the ceramic top coating. Especially, the thermal conductivity of this ceramic coating is induced by the void shapes and networks which are in turn determined by both the spraying conditions and the feedstock material. This study has been performed to optimize the ceramic coating microstructure. First, a hollow zirconia powder was elaborated in the laboratory and compared with a commercial hollow powder. Then, the Taguchi method was performed on thermal spray operating factors to optimize their beneficial effects on the Thermal Barrier Coating (TBC). A new hollow powder with small grains was elaborated using spray drying process. That spray dried powder based on yttria partially stabilized zirconia was optimized using drying simulation tests. It was shown that the formulation and the state of dispersion of the slurry allow to control the powder morphology (from dense to hollow shape). Moreover, for hollow powder, it is possible to vary the thickness of the shell. According to these results, a hollow powder (size ranging from 36 to 130 µm and an 11 µm shell thickness) was elaborated as an original feedstock to produce plasma sprayed TBC. The agglomerated powder was consolidated in an oven and then was compared to a commercial hollow yttria zirconia powder during thermal spray tests. Coating experiments were based on a limited number of operating factors, which have an influence on the deposit microstructure. The seven selected spraying factors concerned the plasma (primary and secondary gas flow rates, arc current), the cooling and the powder deposition (spraying distance, spraying angle, traverse speed). Experiments based on these factors were carried out to elaborate a Taguchi fractional-factorial L16 design. The resulting as sprayed coatings characteristics were quantified with respect to deposition efficiency, roughness (Ra) and porosity (image analysis technique). Through statistical calculation, the parameters that have relevant influence on the coating properties were identified, and their relative importance and some of their interactions were studied. The final aim is to produce an effective thermal barrier coating with a reduced thermal conductivity.
